The present invention relates to a process for purifying chlorosilanes by distillation.
The production of polycrystalline silicon, which is used, for example, in photovoltaics or in the semiconductor industry, starts out from the raw material trichlorosilane (TCS).
This TCS is produced mainly by three different processes.    A) Si+3 HCl→SiHCl3+H2+by-products    B) Si+3 SiCl4+2 H2→4 SiHCl3+by-products    C) SiCl4+H2→SiHCl3+HCl+by-products
In these processes, relatively large amounts of dichlorosilane (DCS) are formed in addition to other by-products or impurities.
Thus, it is known that about 0.1-1.0% of DCS is present in the reaction product of the hydrochlorination of metallurgical silicon as per (A).
The reaction of metallurgical silicon with silicon tetrachloride (STC) and hydrogen (B) generally gives even higher DCS contents in the reaction product, in particular when copper is used as catalyst for this process.
In the hydrogenation of STC as per (C), too, 0.05-1.0% of DCS is found in the reaction product.
DCS is itself a useful product which could be used in the semiconductor industry for the deposition of silicon but also for preparing organofunctional silanes.
However, a very high purity is a prerequisite here. For example, the concentration of boron should be <10 ppta for semiconductor applications.
A further example which may be mentioned is hydrosilylation. In hydrosilylation, derivatives of hydrosilanes are reacted by a catalytic addition reaction onto vinyl groups or other multiple bonds. Typical catalysts are complexes of the noble metal platinum. Here, the concentration of boron should be <1 ppbw since boron acts as a catalyst poison.
DCS from the abovementioned processes A-C is unsuitable for these applications since, in particular, the boron contents are too high.
Since boron is mainly present as BCl3 having a boiling point of 8.3° C. and has a boiling point similar to that of DCS (boiling point 12.5° C.), boron is concentrated virtually completely in the DCS product stream in the subsequent distillation.
Despite a difference in boiling point of just about 30 K, separation of BCl3 from TCS by distillation is incomplete, particularly when boron contents of <0.1 ppm in the TCS are to be achieved.
In the prior art, the BCl3 produced in the hydrochlorination of metallurgical silicon is discharged together with an amount of trichlorosilane from the system. This is described, for example, in “Handbook of Semiconductor Silicon Technology”, William C. O'Mara, Robert B. Herring and Lee P. Hunt, Noyes Publications, USA 1990, see page 4, fig. 2.
Because of the very similar boiling point, DCS is also discharged together with BCl3 from the system, which leads to poorer economics of the overall plant.
Essentially four different approaches are known for separating boron impurities from TCS.
Thus, purely distillative processes and also processes having a hydrolysis, complexation or adsorption step have been described.
DE 10 2007 014 107 A1 describes a process for obtaining boron-depleted chlorosilanes from a boron-containing chlorosilane mixture by removal of a boron-enriched distillation stream by distillation, with a boron-enriched side stream being branched off from at least one distillation column of an arrangement of one or more distillation columns and disposed of or passed to another use. Various column arrangements and taking off of products from overhead and side offtakes on the respective columns enables the boron content in the pure DCS in substreams to be reduced to about 50 ppm. However, the boron concentration is increased even more greatly in another substream containing DCS and TCS. A further disadvantage is that a not inconsiderable amount of TCS is lost as waste.
DE 10 2008 002 537 A1 discloses a process for reducing the boron content in composition I comprising at least one silicon halide, in which process the composition I is, in a first step, brought into contact with up to 600 mg of moisture per kilogram of the composition I, the composition I which has been brought into contact with moisture from the first step is optionally entirely or partly passed at least once to a substep for separating off hydrolyzed boron- and/or silicon-containing compounds to give a prepurified composition II which is entirely or partly returned to the first step or fed to a second step of the process, where hydrolyzed boron- and/or silicon-containing compounds are separated off by distillation in the second step to give a prepurified composition II having a reduced content of boron as distillate.
The boron content in chlorosilanes can thus be reduced by, for example, adding water in a suitable form. Reaction of boron halide with water forms higher-boiling hydrolysates which can be separated more easily from chlorosilane by distillation. However, these processes require an additional purge stream in order to separate off the boron and chlorosilane hydrolysates formed (e.g. >5% purge stream based on the starting material). Deposition of silica in plant components and corrosion due to HCl formed are also problematical. The corrosion subsequently leads to liberation of dopants such as P and As from the steel of the plants.
EP 2 036 858 A2 claims a process in which boron- and phosphorus-containing chlorosilanes are brought into contact with the complexing agent benzaldehyde and oxygen. As a result of oxidation and complex formation, the boron compounds present in the chlorosilane can be separated off easily. However, as described in example 6 of this patent application, about 10% of residues with which the boron complex has to be discharged are obtained. Owing to the relatively slow reaction (30 min), this process is not suitable for continuous operation. In addition, the outlay in terms of apparatus is increased by a stirred vessel and the introduction of organic contamination into the target product is probable.
DE 10 2008 054 537 describes a process for treating a composition containing at least one silicon compound and at least one foreign metal and/or a compound containing a foreign metal, in which the composition is, in a first step, brought into contact with at least one adsorbent and/or at least one first filter and is optionally, in a further step, brought into contact with at least one filter to give a composition in which the content of the foreign metal and/or the compound containing a foreign metal is reduced.
Here, the boron content in chlorosilanes is reduced by contacting with water-free adsorbents. However, very large amounts of adsorbent (120 g/250 ml of TCS) are required in order to achieve the desired purification effect. This makes the process uneconomical, especially since a continuous process is not really feasible, which is an economic disadvantage in the production of chlorosilanes in semiconductor quality. The use of adsorbents also requires further apparatus (e.g. filtration) and incurs the risk of introducing other impurities into the semiconductor-pure product.
In the light of the problems described, it was an object of the invention to purify contaminated chlorosilanes with a reduced outlay and accumulate and discharge the impurities in ideally small purge streams. On the present-day economical scale, the yield of TCS has to be significantly above 95%.
It has been found that purely distillative processes are advantageous since no additional apparatus is required and these processes can be operated continuously in a simple way. The losses of chlorosilanes can be best minimized in these.
An advantage of distillation processes is the fact that the risk of introduction of further impurities is very low.